A lithium ion cell having a high energy density and a high operating voltage has rapidly come into wide use as a power source for mobile or portable devices, such as portable phones, notebook-size personal computers and video cameras. Further, various researches for practical application to a satellite, a rocket, an electric vehicle and a nighttime-electric-power storage system are being conducted.
A lithium ion cell employs a carbon-based material and a lithium transition metal oxide, such as LiCoO2, respectively, in negative and positive electrode, and has an operating voltage of 4 V or more. Thus, an electrolyte solution for lithium ion cells is required to have electrochemical stability even in an operation at 4 V or more. A nonaqueous electrolyte solution prepared by dissolving an electrolyte, such as lithium fluorophosphate (LiPF6), in a mixture of carbonate-based nonaqueous solvents, such as ethylene carbonate (EC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) has been developed to meet such a requirement, and put into general use.
However, due to relatively low flash or inflammation points of these nonaqueous solvents, the nonaqueous electrolyte solution involves a problem about safety, firing or explosion likely to be caused by a wrong operation, such as short-circuiting, overcharge or over-discharge. As measures against this problem, it has been proposed to prepare a nonaqueous electrolyte solution using a mixture of a fluorinated solvent and an organic phosphate compound having no flash point. For example, Japanese Patent Laid-Open Publication Nos. 2000-235867 and 2002-280061 disclose the use of an organic phosphate compound, such as trimethyl phosphate or triethyl phosphate. However, if this organic phosphate compound is applied to a lithium ion cell, it will be reductively decomposed on a surface of a carbon-based negative electrode to preclude charge-discharge functions as a cell. While the charge-discharge functions can be maintained by mixing an inflammable solvent, such as carbonate-based solvent, with the an organic phosphate compound solvent, while limiting a ratio (volume ratio) of the organic phosphate compound in the mixed solvent to 50% or less, a primary solvent will consist of a low-flash-point solvent, such a carbonate-based, lactone-based, ether-based, sulfolane-based or dioxolan-based solvent, resulting in loss of non-flammability as an electrolyte solution.
Japanese Patent Laid-Open Publication Nos. 2000-348762 and 2000-215911 discloses the use of a fluorinated solvent. This fluorinated solvent has a problem about no applicability to a 4 V-class lithium ion cell, due to poor oxidation/reduction resistances and low solubility relative to lithium salts.
Japanese Patent Laid-Open Publication No. 2002-203597 discloses a technique of adding vinylene carbonate and/or vinyl ethylene carbonate into a phosphate ester-based electrolyte solution to suppress reductive decomposition of an organic phosphate compound. However, it is practically difficult to fully eliminate reductive decomposition of an organic phosphate compound based on the addition of the two compounds, and an obtained cell is not insufficient in terms of practical usefulness, due to poor charge/discharge characteristics.
As above, from a standpoint of improving both charge/discharge characteristics and safety in a cell, there is a strong need for developing a new mixed additive and employing an optimal type and concentration of lithium ion salt to obtain a non-flammable nonaqueous electrolyte solution capable of suppressing reductive decomposition of phosphate ester on a surface of a carbon-based negative electrode so as to achieve a lithium ion cell having enhanced charge/discharge characteristics.